FIG. 1 is a somewhat schematic plan view of a conventional substrate processing tool 11 of the type which is used to apply semiconductor manufacturing processes to substrates such as silicon wafers or glass plates. As is familiar to those who are skilled in the art, silicon wafers may be used for fabrication of semiconductor devices such as processors, memory devices, etc. Glass plates may be processed to manufacture flat panel displays for use as computer monitors, television displays or the like.
The processing tool 11 includes a centrally located transfer chamber 13, to which process chambers 15 are coupled. In each of the chambers 15, a semiconductor fabrication process such as thin film deposition, oxidation or nitridization, etching, or thermal or lithographic processing, may be performed. Substrates to be processed in the processing tool 11 are introduced into the processing tool 11 via at least one load lock chamber 17 coupled to the transfer chamber 13. A substrate handling robot 19 is installed in the transfer chamber 13 to transfer substrates among the load lock chamber 17 and the process chambers 15.
It is known to equip a transfer chamber of a processing tool with sensors for detecting whether a substrate is present and is properly positioned for loading into one of the process, load lock, or other chambers coupled to the transfer chamber. One conventional sensor system for detecting the presence of a transparent substrate, such as a glass plate, is illustrated in FIG. 2.
FIG. 2 is a side schematic view of the transfer chamber 13 of FIG. 1. For convenience, only a top (lid) 21 and a bottom 23 of the transfer chamber 13 are shown. The transfer chamber 13 includes a sensor system 25 comprising a transmitter/receiver unit 27 and a reflector 29 coupled to the bottom 23 and top 21, respectively, of the transfer chamber 13. As is conventional, the transmitter/receiver unit 27 may be employed to transmit a light beam 31 toward a transparent substrate S (e.g., a glass substrate for a flat panel display). The transmitted beam 31 travels through the glass substrate, and reflects off of the reflector 29 so as to form a reflected beam 33. The reflected beam 33 travels back through the substrate S and is detected by the receiver portion (not separately shown) of the transmitter/receiver unit 27. Note that the spacing between the transmitted and reflected beams 31, 33 is exaggerated for clarification purposes (e.g., as the spacing between the transmitting and receiving portions of the transmitter/receiver unit 27 may be only about a quarter of an inch or less).
When the substrate S is not present between the transmitter/receiver unit 27 and the reflector 29, the reflected beam 33 which is detected by the transmitter/receiver unit 27 has approximately the same intensity as the transmitted beam 31. However, when the substrate S is present between the transmitter/receiver unit 27 and the reflector 29, the reflected beam 33 is attenuated by each pass through the substrate S (e.g., due to absorption, scattering, etc., by the substrate S); and the reflected beam 33 which is detected by the transmitter/receiver unit 27 has a smaller intensity than the transmitted beam 31. Accordingly, the presence/absence of the substrate S may be deduced based on the intensity of the reflected beam 33 (relative to the transmitted beam 31) detected by the transmitter/receiver unit 27.
Although such a conventional sensor arrangement should perform satisfactorily for its intended purpose, it has been found that such an sensor arrangement may occasionally produce false readings. Specifically, it has been found that the sensor system 25 may erroneously indicate that the substrate S is not present. Accordingly, an improved sensor system would be desirable.